In conclusion of the preparation of human HIV-1 neutralizing sera as reference reagents for the World Health Organization (WHO), a summary of the characterization tests results and of an international collaborative study were sent for approval to Dr. Saladin Osmanov in the Vaccine Development Unit for the Global Programme on AIDS in Geneva, Switzerland. In December of 1992, a WHO/GPA Steering Committee on Vaccine Development met and decided that the results of uniformity of fill, moisture content, potency, and reproducibility did not fully meet the strict criteria necessary to be a WHO standard, but they recognized the importance of the availability of such well-characterized reagents to standardize measurement of anti-HIV neutralizing antibodies, and recommended these reagents as Reference Reagents for research purposes as part of the WHO AIDS Reagent Project. There are approximately 2500 lyophilized, glass sealed ampules of each of the two reference samples and arrangements were made to label and send 200 aliquots of each reference sera to each of the three Repositories of the WHO AIDS Reagent Project, the first being the AIDS Research and Reference Reagent Program in Rockville, MD, USA, a second at the National Institute of Biological Standards and Control in Hertfordshire, England, and the third at the Institut Cochin de Genetique Moleculaire in Paris, France. These reagents are now available for distribution to interested researchers. Presently a manuscript is in preparation that will explain the origin of these samples, the need for such reagents, and show the results of all of the characterization tests and the international collaborative study in which six laboratories in the US and Europe participated.